1. Field of the Invention
The present invention relates to a manufacturing method of a thin film device substrate which comprising the step of transferring a thin film device formed on one substrate onto another substrate.
2. Description of the Related Art
For the switching element in the pixel section of the liquid crystal display unit, the thin film transistor (referred to as the “TFT” hereinafter) has been being widely used. In recent years, with the object of achieving the liquid crystal display unit of higher precision as well as producing the system on glass (referred to as the “SOG” hereinafter) successfully, there have been growing demands for higher operation speeds. The SOG herein is a technique to integrate, along with the TFT for driving the liquid crystal, the TFTs in adjacent circuits such as the driver circuit, the memory and the CPU (Central Processing Unit) onto one glass substrate.
When, for example, the liquid crystal display device making use of the TFT is fabricated, the glass substrate is generally utilized. The glass substrate is, however, not necessarily the best material. The glass substrate is disadvantageously heavy, fragile and inflexible. As against this, as the substrate which is lighter, less fragile and more flexible, there can be given the plastic substrate. Nevertheless, the TFT formed on the plastic substrate has the disadvantage of poorer performance than that of the TFT formed on the glass substrate, since the processing temperature for the TFT formed on the plastic substrate is limited to 150° C. or so. Accordingly, there have been brought forward several methods wherein, after a plastic substrate is bonded onto a TFT formed on a glass substrate, the glass substrate is peeled off therefrom and thereby the TFT is transferred onto the plastic substrate. They are further described below.
In a method disclosed in Japanese Patent Application Laid-open No.288522/1996, by forming numerous trenches in a glass substrate and then forming a silicon oxide film on the glass substrate by means of spin-coat, formation of gaps in the trenches is made, and following the TFT formation, an etching solution is permeated into these gaps, and thereby the glass substrate is peeled off. In a method disclosed in Japanese Patent Application Laid-open No. 26733/1999, an isolation layer made of amorphous silicon or the like is laid on a glass substrate that can transmit a laser light, and thereon a TFT is formed. After another substrate is adhered onto this TFT, backside irradiation with the laser light is applied thereto, whereby the bonding force of the isolation layer is weakened so as to separate the glass substrate therefrom. In a method disclosed in Japanese Patent Application Laid-open No. 212116/1999, a glass substrate is characteristically removed by chemical polishing.
In the conventional techniques, however, the following problems are known. In the method of Japanese Patent Application Laid-open No. 288522/1996, the step of forming trenches in a glass substrate is comprised so that the glass substrate surface becomes rough, which lowers the pattern accuracy of the thin film device formed on the glass substrate. In the method of Japanese Patent Application Laid-open No. 26733/1999, because the peeling-off is made with a laser, a material such as a glass having a low transmittance for the laser light or a silicon impervious to laser light cannot be employed for the substrate. In the method of Japanese Patent Application Laid-open No. 212116/1999, the use of the polishing method lengthens its processing tome period.
Accordingly, an object of the present invention is to provide a method of manufacturing a thin film device substrate wherein trench fabrication is not required on the substrate surface, wherein a material which is impervious to light can be used, and wherein the substrate can be peeled off in a short period of time.